1. Field of the Invention
The present invention relates to a method of forming a nitride semiconductor having improved quality of a metal nitride thin film via surface treatment capable of reducing dislocations in the thin film during growth of the metal nitride thin film.
2. Description of the Related Art
In recent years, replacement of conventional illumination with high efficiency semiconductor illumination has been promoted due to increased concern over energy issues relating to depletion of fossil fuels. Further, under the fierce competition for high-speed information processing technology, an attempt for application of gallium nitride (GaN) to high electron mobility transistors (HEMT) or power switch devices operating at high speeds has also been actively made in the field of information technology.
A GaN thin film having a Wurtzite structure has a direct transition type band gap of 3.4 eV at room temperature, and can be usefully applied to light emitting diodes (LEDs) and laser diodes (LDs) that emit light in the range of blue color and ultraviolet rays.
Particularly, the GaN thin film permits the formation of continuous solid solutions along with indium nitride (InN) and aluminum nitride (AlN), which have the Wurtzite structure like the GaN thin film and band gaps of 1.9 eV and 6.2 eV, respectively.
The GaN thin film permits wavelength adjustment according to an active energy and doping concentration of an impurity, and forms ternary nitrides depending on composition to facilitate manufacture of visible light emitting diodes having a wide range of wavelengths, so as to be applicable to a wide range of applications.
However, regardless of the wide range of applications of the GaN thin film, the properties of the GaN thin film make it very difficult to fabricate the GaN thin film into a bulk-type single crystal substrate such as an ingot.
Currently, the GaN thin film is formed through epitaxial growth on a substrate by Metal Organic Chemical Vapor Deposition (MOCVD).
Here, since the GaN thin film is generally formed through hetero-epitaxial growth rather than homo-epitaxial growth, selection of an appropriate substrate is critical.
In particular, a sapphire (α-Al2O3) substrate and an SiC substrate are generally employed as heterogeneous substrates for use in growth of the GaN thin film. However, lattice mismatches of 16% and 3.5% between the respective heterogeneous substrates and gallium nitride for the a-axis cause mismatch dislocation, which is created from an early stage of thin film growth, and other defects, such as threading dislocation, stacking fault, inversion domain boundary, and the like.
Since such defects are very important factors in determining lifespan and luminescence efficiency of diodes, various attempts have been made to remove or suppress the defects.
For example, a buffer layer is conventionally used to prevent the formation of defects. Typically, a buffer layer of aluminum nitride or gallium nitride is used. That is, such a non-crystalline or polycrystalline buffer layer provides many nucleation sites, which have the same crystallinity as those of the substrates, thereby facilitating two-dimensional growth of gallium nitride while promoting lateral growth thereof through reduction in interfacial energy between the thin film and the substrates.
However, since the buffer layer of aluminum nitride or gallium nitride is formed by nitride treatment through MOCVD or molecular beam epitaxy before crystal growth of the gallium nitride thin film, such nitride treatment causes surface roughening of the gallium nitride thin film according to treatment duration, thereby deteriorating quality of the gallium nitride thin film.
In other words, since non-crystalline compounds are generated to form protrusions on the surface of the sapphire substrate subjected to nitride treatment, it can be understood that the conventional process provides different results depending on whether process conditions are optimized or not. Thus, the conventional method inevitably requires very careful control of the process upon crystal growth of gallium nitride.
To solve such problems, a technique for Epitaxial Lateral Overgrowth (ELOG) of a gallium nitride crystal has been studied. For epitaxial lateral overgrowth of the gallium nitride crystal, a mask having a periodic pattern is formed on a substrate or a GaN buffer layer, followed by growing gallium nitride to a thickness of the mask layer or more on a region of the substrate where the mask is not formed, that is, on a window region, and laterally growing a gallium nitride thin film over the mask. The gallium nitride film formed by the epitaxial lateral overgrowth has a significantly reduced density of threading dislocations, thereby enhancing chip performance.
However, the epitaxial lateral overgrowth generally requires re-growth. For example, when forming the gallium nitride thin film on the sapphire substrate through the epitaxial lateral overgrowth, it is above all necessary to form a gallium nitride buffer layer at a low temperature. When the gallium nitride buffer layer is grown at low temperatures, the GaN buffer layer is undesirably grown on the surface of the mask as well as on the window region, thereby providing an obstacle in epitaxial lateral overgrowth.
Accordingly, in the conventional technique it is necessary for the epitaxial lateral overgrowth to be performed after growing the mask on sapphire substrate/low temperature GaN/high temperature GaN layers. Further, an insulation layer used as the mask is likely to generate stress in the GaN thin film and is a potential impurity, which can act as a contaminant. Moreover, if nucleation occurs at a high speed on the mask, some of the GaN layer may not meet the other GaN layer on the mask during the epitaxial lateral overgrowth, thereby requiring an additional process condition for achieving rapid epitaxial lateral overgrowth.